Jet weaving machines are usually equipped with a series of nozzles which can be sequentially actuated for guiding a weft yarn through the shed. The respective length of the weft yarn inserted in the course of one weft yarn insertion shot is determined by a yarn feeding and measuring device adapted for intermittently storing a necessary quantity of yarn for the weft yarn insertion on a storage drum thereof. The feeding device serves not only for intermittently storing the yarn, but is also for terminating the weft yarn insertion procedure by actuating a stopping device for preventing any further withdrawal of yarn from the storage drum of the feeding device. Feeding devices are known per se in the art. Feeding devices adapted for jet looms are, for example, known from the European patent application No. 83 109 818.1-2304 (and corresponding U.S. Pat. No. 4,627,474), which is owned by the applicant. The content of this prior application of the applicant is incorporated into the present application by this cross-reference.
Jet looms usually have an opto-electric sensor unit arranged at one end of the shed for detecting the arrival of the weft yarn. The sensor is used for checking whether the weft yarn has arrived at the other end of the shed, which indicates that the weft yarn insertion is terminated. The yarn arrival sensor can also be used for detecting any weft yarn insertion faults, like a weft yarn breakage in the ccurse of the insertion of the weft yarn into the shed, or other faults which can be detected by checking whether a sensor signal is generated at the end of each weft yarn insertion cycle or not. Prior art looms making use of an arrival sensor are, for example, known from U.S. Pat. No. 4,270,579 and from the European patent application EP-A- No. 164 773. The content of these prior art references are alos incorporated herein by cross-referring to these references.
As indicated above, the prior art devices make use of the sensor signal as generated by the opto-electric arrival sensor, so as to survey whether the weft yarn has been correctly inserted into the shed of the jet weaving machine or jet loom. A missing signal indicating the arrival of the weft yarn is an indication that some fault has occurred, which fault prevents the forward end of the yarn from arriving at the other end of the shed. A possible fault of this kind is, for example, a yarn breakage. This information concerning the non-arrival of the weft yarn at the end of the shed is fed to a control unit for stopping the operation of the loom if this specific fault occurs. Moreover, a weft yarn arrival at the end of the shed which takes place too early, is also an indication of the erroneous weft yarn insertion. Thus, an arrival sensor signal which is generated too early is also an indication of a weft yarn insertion fault. Hence, a too short insertion time between the departure of the weft thread at the insertion end of the shed, and the arrival of the head of the weft yarn at the other end of the shed, is also used for interrupting the operation of the jet loom. Moreover, the timing of the arrival of the weft yarn head at the arrival sensor is often used for adjusting other loom operations, like the timing of the respective actuations of the jet nozzles for adapting it to the yarn movement.
It has turned out that the arrival sensor tends to generate erroneous arrival signals. This misbehaviour is caused, for example, by particles of dust and lint in the detection area of the arrival sensor between the light source thereof and the photoelectric detection element thereof. When reducing the sensitivity of the arrival sensor for preventing an erroneous detection of the weft yarn arrival, caused by dust or lint in the detection area, the tendency to non-detect the arrival of light and thin yarn ends increases. Hence, the adjustment of the sensitivity of the arrival sensor must be chosen such that the sensitivity versus erroneous detections caused by dust or lint is lowered, while still having a sufficient sensitivity for detecting the arrival of yarn at the arrival end. Nevertheless, a certain percentage of mis-detections could not be avoided and resulted in an erroneous interruption of the weaving process.
In view of this state of art, the present invention is based on the technical task, to further enhance a device surveying the insertion of a weft yarn in a shed of a jet loom, comprising an arrival sensor for detecting the arrival of the weft yarn at one end of the shed of the jet loom, so that the reliability of the detection of the arrival of the weft yarn is increased.
In accordance with the present invention, the arrival sensor, and at least one yarn sensor, located in the vicinity of a withdrawal end of a storage drum of the yarn feeding device, are connected to a survellance circuit for generating a signal indicating the completion of the insertion of the weft yarn into the shed. The yarn sensor located in the vicinity of the withdrawal end of the storage drum serves to generate a pulse signal indicating that the yarn passes the sensor detection area during the withdrawal of the yarn from the storage drum. Hence, the pulse signal can be used for detecting the actual velocity of the yarn withdrawal from the drum. The surveillance circuit measures the period of time lapsed since releasing a stopping device at the beginning of the weft yarn insertion cycle and periodically adjusts the measured period of time on the basis of the yarn sensor signal. Hence the measured period of time is continuously adjusted to the dynamic weft yarn withdrawal operation. Thus, the dynamically corrected measured period of time is an optimal representation of the actual position which the weft yarn had. The surveillance circuit only generates a signal indicating the completion of the weft yarn insertion, if two conditions are simultaneously fulfilled:
(1) the measured and corrected period of time exceeds a predetermined period of time between 80% and 99.5% of the period of time required for a complete weft yarn insertion cycle, and
(2) an arrival sensor signal is generated.
Alternatively, the dynamically adjusted value as periodically measured and corrected by the surveillance circuit can be a calculated length of the weft yarn withdrawn from the storage drum, 7, instead of the measured and corrected period of time. Advantages and details of the design of the respective devices in reference to the drawing.